The present invention is directed to power control systems for electric traction motor propelled vehicles and, more particularly, to an accelerator position responsive control system capable of reacting to each increment of accelerator movement throughout the full range of movement.
The response of an internal combustion engine in a vehicle to accelerator movement tends to be substantially linear, i.e., the engine RPM varies, under constant load, for each increment of movement of the accelerator, with maximum torque being developed with maximum accelerator displacement. An operator switching from an internal combustion engine powered vehicle to a direct current traction motor powered vehicle could more easily adjust to the electric vehicle if the accelerator response were similar. However, such is not normally true.
In present day electric vehicles, electronic power regulators are used to control the torque, or speed, developed by the electric traction motors. Typically, the regulator comprises a time-ratio or chopper circuit which varies the power developed by the motors by controlling the percentage of time that the motors are connected directly to a power source. For maximum mobility, the power source is a battery, which limits the available power to the motors. The regulator also includes apparatus responsive to accelerator position for varying the mark-space ratio of the chopper circuit.
In some systems the accelerator position is translated directly into mark-space ratio, i.e., mid-position corresponds to 50 percent conduction of the chopper circuit and full displacement corresponds to 100 percent conduction. However, at very low speeds, such a system produces very high torques with small accelerator displacement since the counter electromotive force of the motor is speed dependent. In practice, such systems result in available torque control at low speed being confined to the first 30 percent of accelerator movement tending to make the accelerator very sensitive.
In some other electric vehicle systems motor current is made directly proportional to accelerator position. This latter type of system has good performance at low speeds; however, at higher speeds where the application of full voltage to the motor results in a low value of current due to the large counter EMF, the maximum value of current may be obtained with as little as one-sixth of the total pedal displacement.
It is an object of the present invention to provide an improved torque/speed regulator for an electric motor. It is another object of the present invention to provide an improved torque/speed regulator for an electric motor powered vehicle which regulator is responsive to incremental accelerator movement over the full accelerator displacement range.